1. Field
This invention relates to a process and attendant apparatus for use in processing metal-containing ores by use of a biological (hereinafter "bioleaching") technique. More particularly, this invention is directed toward a process and apparatus for use in processing precious metal-bearing pyrite ore concentrates which are not efficiently leachable by conventional processes and means, such as leaching using cyanide solutions.
2. State of the Art
Recent interest in the metallurgical field has focused on the use of special types of autotrophic bacteria, e.g. thiobacillus ferrooxidans and thiobacillus thiooxidans, in treating sulfide ores and concentrates. The use of such bacteria in heap leaching treatments to solubilize copper from low-grade ores has been known for several decades.
Currently, however, the interest in applying this biochemical technology has been focused on continuous processes to treat sulfide concentrates. These continuous processes either make the concentrates more susceptible to conventional cyanide leaching or actually extract the desired metal from the concentrate.
Particular attention has been focused on gold-bearing, silver-bearing, or platinum-bearing pyrites and arsenopyrites that are, at best, marginally susceptible to cyanide solution leaching. These concentrates' insusceptibility to cyanide leaching is due to the desired metals, e.g. gold or silver, being encapsulated by the pyrite crystal. The pyrite crystal is insufficiently porous to allow penetration of the cyanide solution for a metal-cyanide dissolution reaction to take place. Comminution of the metal-bearing pyrite, in itself, does not expose sufficient metal values to be economically feasible inasmuch as greatly increased cyanide solution and energy consumption are required. The above-described bacteria can, however, induce the biooxidation of sulfide and iron in the unsolubilized pyrite crystal, leaving the gold, silver or platinum intact. The resulting residue, after separation of the soluble biooxidation products, is amenable to metal extraction employing conventional cyanide, thiourea, or thiosulfate solution leaching techniques. On occasion, even a partial biooxidation of the metal-bearing pyrite by the above-described bacteria is sufficient to allow successful cyanide solution leaching of the resulting residue.
The described process is adaptable to the leaching of other metals. For example, chalcopyrite can be leached for its copper content, and zinc sulfides can be leached to produce zinc sulfate solutions (ZnSO.sub.4). Other elements present as sulfides may also be solubilized, such as antimony and arsenic.
The current processes using the above-described bacteria for solubilizing the metal-bearing sulfide ores and concentrates are very energy intensive. The chemical reaction used by these bacteria is oxidation. Hence, oxygen transfer is a key step in the process.
Approximately an equal weight of oxygen is required to oxidize pyrite. The systems currently employed in the art require one horsepower hour per approximately 2.5 to 4 pounds of oxygen transferred into liquid phase.
Consequently, to oxidize one ton (2,000 pounds) of concentrate, these systems consume approximately 400 to 600 kilowatt hours (KWH) of energy.
Metallurgical processing by leaching typically employs a number of tanks operating in series, each tank overflowing or cascading into a subsequent tank. The total retention time in the circuit (i.e., the series of tanks) is that required for processing. Reagents required for leaching are usually added to the first tank, and if necessary, to subsequent tanks. With bioleaching, there is a significant time required for bacterial growth to reach a level of suitable bioactivity. Simply adding bacteria to the first tank will not immediately provide sufficient numbers of microorganisms to achieve any great degree of processing. Furthermore, as the pulp flows from one tank to the next and the bioreaction continues, the amount of soluble by-product material produced can become very high. Soluble by-product material, e.g., metal sulphates, sulfuric acid, and arsenic acid, is a product of the bioleaching operation, which if present in the reaction tank in excessive proportion inhibits the speed of the reaction. Thus, without selective removal of this soluble by-product material, the reaction rate is diminished and the process is slowed.
One of the critical problems involved in developing a workable process is the transfer of nutrients and oxygen into the tanks in sufficient quantities so as to be readily assimilated by the bacteria. The bacteria require a supply of nitrogen, potassium, phosphorus and carbon dioxide as nutrients. These nutrients are typically provided by adding ammonium sulphate, potassium, phosphates and gaseous carbon dioxide to the tanks. Problems associated with transfer of the oxygen are distinguishable from those encountered providing nutrients and carbon dioxide. Since oxygen transfer is critical and the quantity required is very large, this part of the process is of paramount importance to overall process cost and performance. The method practiced conventionally involves injecting large quantities of oxygen directly into the solution and providing a mixing means whereby the oxygen is dispersed or distributed within the solution. These processes involve introducing the oxygen and transferring it from a gas phase into an aqueous phase, i.e., dissolving it within the solution.
The method conventionally adopted to effect this transition typically utilizes turbines which are placed within the slurry and rotated at high speeds. Though the turbine action does provide considerable mixing action, i.e., dispersion the oxygen, within the solution; the rotation of the turbines also produces cavitation effects. These effects cause the air bubbles within the solution to be forcedly aggregated into larger air masses or bubbles due to the vacuum effects and turbulence attendant the action of the turbine blades. Resultingly, the turbines, though functioning to disperse the air within the slurry, also function to create large air masses or bubbles which have a relatively small surface area to volume ratio. A basic problem confronting the conventional technology is the power requirement requisite to operate the turbines. The turbine power is that required to turn the blades at a sufficient velocity to achieve the desired quantity of oxygen being introduced into the aqueous phase of the solution. Oxygen in this phase may be readily assimilated by the bacteria. A considerable mixing action is required, necessitating a high tip speed on the turbine rotor blades. Understandably, this high tip speed is only obtained by an infusion of considerable quantities of energy into the turbine itself.
A second problem confronting the current technology is the removal of soluble by-product matter produced within the solution by the reactions effected or initiated by the presence of the bacteria. One typical approach to this problem is the use of a thickener. The slurry is admitted into the thickener and soluble components are removed via the overflow of the slurry/thickener mixture. This approach generally results in the bacteria, which are suspended within the liquid phase, being carried away together with the soluble matter, in the overflow. This removal of bacteria from the slurry slows the process reaction rate. Furthermore, the slurry thickener mixture is not aerated during the separation of the soluble material from the slurry. Therefore, the bacteria which remain with the solids are deprived of requisite oxygen and resultingly tend to slow their activity and further delimit the rate of the process.
A third major problem of the conventional process is the length of overall retention time required to achieve a desired extent of biooxidation. Systems currently employed require a retention time of many days. The retention time is inversely proportional to reaction rate, which is found to be enhanced by maximization of oxygen and nutrient supply. The reaction rate is delimited by the presence of reacted products and by-products in the reactor vessel and by the loss of biomass (i.e., microorganisms or bacteria) to the reactor effluent.
Failure of the current art to address effectively the above aspects of bioleaching has resulted in current bioreactors and processes being marginally efficient in both cost and process performance.